Process for extracting pure, coarse grain silicic acid crystals from spent lye

Abstract

A process is disclosed for extracting pure, coarse-grain silicic acid crystals from silicic acid-containing spent lye in cellulose production, comprising adding coarse-grain silicic acid to alkalized spent lye, lowering the pH of the alkalized spent lye to about 9 such that a sediment is formed, separating the sediment formed into coarse grain and fine grain silicic acid with little lignin and lignin-containing spent lye free from silicic acid, and separating the coarse grain and fine grain silicic acid with little lignin into course grain silicic acid and lignin. At least part of the course grain silicic acid separated is fed back to the alkalinized spent lye to be desilicified.

I claim: 
     
       1. A process for extracting pure, course-grain silicic acid crystals from silicic acid-containing spent lye in cellulose production, comprising: (a) adding coarse-grain silicic acid to alkalized spent lye;   (b) lowering the pH of the alkalized spent lye to about 9 such that a sediment is formed in the spent lye;   (c) separating the sediment formed in accordance with step (b) into a mixture of (i) coarse and fine grain silicic acid and lignin and (ii) lignin-containing spent lye free from silicic acid;   (d) separating coarse grain silicic acid from mixture (i) of step (c); and   (e) separating lignin from mixture (i) to provide pure, coarse-grain silicic acid.   
     
     
       2. The process as in claim 1 where at least part of the separated coarse grain silicic acid from step (d) is added to the alkalized spent lye in step (a). 
     
     
       3. The process as in claim 2 where the at least part of the separated coarse grain silicic acid is added discontinuously to the alkalized spent lye. 
     
     
       4. The process as in claim 1 further comprising adding the lignin from step (e) to lignin-containing spent lye free from silicic acid to provide a fuel composition. 
     
     
       5. The process as in claim 1 in which the pH of the alkalinized spent lye is lowered successively by exposing the alkalized spent lye in a first vessel to a gas comprising CO 2  and transferring the alkalized spent lye from the first vessel to a second vessel and exposing the alkalized spent lye in the second vessel to a gas comprising CO 2 . 
     
     
       6. The process as in claim 1 in which the mixture of coarse grain and fine grain silicic acid and a small amount of lignin is washed such that coarse grain silicic acid is obtained, further comprising feeding said coarse silicic acid to the spent lye prior to step (c). 
     
     
       7. The process as in claim 6 in which a material selected from the group of a carbonate and a hydroxide is added to the coarse grain silicic acid to the spent lye prior to step (c). 
     
     
       8. The process as in claim 6, in which silicic acid obtained in an overflow of the washing is added to the spent lye to be desilified prior to step (c) of claim 1. 
     
     
       9. The process as in claim 1, in which the pH value of the spent lye during the sedimentation of the spent lye is approximately the pH value of a relative silicic acid oversaturation of the spent lye of less than 3. 
     
     
       10. The process as in claim 5, in which the silicic acid concentration in all precipitation steps is maintained above 10 g/l through backfeeding of coarse crystal silicic acid and in that the silicic acid precipitated per hour in the step is equal to approximately 10% of the total amount of silicic acid added to the spent lye prior to step (c) of claim 1. 
     
     
       11. The process as in claim 1, further comprising categorizing and washing fine grain silicic acid and lignin and adding the lignin to the desilified spent lye while the fine grain silicic acid is added to the spent lye to be desilified. 
     
     
       12. The process as in claim 1, in which removal of the coarse silicic acid takes place throughout one or several washing and separation steps in opposing flow, whereby an increase of the pH value is carried out in a first washing step in a filter with wash water and in that the wash water overflow of the last separation step with a low content of dry substance is introduced into a fiber line of the cellulose production and in that the wash water of the first separation step is introduced into the spent lye to be desilified. 
     
     
       13. The process as in claim 12, where hydrocyclones, centrifuges and/or washing filters are used for the washing and separation, and in that the washed silicic acid slurry containing more than 300 g/l of dry substances is collected in a dewatering container and in that the wash water produced is fed back into the washing step. 
     
     
       14. A process for separating coarse-grain, pure silicic acid crystals from silicic acid-containing spent lye in cellulose production, whereby the spent lye is alkalized to a pH of at least 11 through the addition of lye and whereby the desilicification, through lowering the pH by exposure to a gas comprising CO 2  is carried out in at least two precipitation vessels until the desired residue of silicic acid is obtained and the precipitated silicic acid is separated from the spent lye by sedimentation, characterized in that the alkalized spent lye is inoculated with coarse-grain silicic acid and its pH is then lowered to about 9, and in that sludge produced as the pH is lowered to about 9 is separated during its sedimentation into coarse grain and fine grain silicic acid with a small mount of lignin and into lignin-containing spent lye free of silicic acid, and in that the coarse grain silicic acid is separated from the lignin and the separated lignin is added to the lignin-containing spent lye. 
     
     
       15. The process as in claim 13 in which the coarse grain silicic acid is separated from the lignin in a washing device.